The 'grandmother hypothesis' was proposed by Professor Kristen Hawkes -- co-author on this latest study -- and colleagues in 1997 based on their observations in the 1980s of the Hazda hunter-gatherer people in Tanzania.

They noted that older women of the tribe spent their days collecting foods for their grandchildren. Except for humans, all other primates and mammals collect their own food after weaning.

Hawkes, from the Department of Anthropology at the University of Utah, proposed that when grandmothers helped feed their grandchildren after weaning, their daughters could produce more children at shorter intervals.

They then used computer modelling to show that by allowing their daughters to have more children, those ancestral females who lived long enough to become grandmothers passed their longevity genes to more descendants, who had longer adult lifespans as a result.

The team's computer simulations showed from a start point of just 1 per cent of women living to grandmother age within 24,000-60,000 years about 43 per cent of adult women are grandmothers -- a figure consistent with today's hunter-gatherer populations.

For the latest study, Hawkes and her team again used computer modelling to examine how the evolution of grandmothering impacted on male to female interaction.

For the study they compared simulations of male to female sex ratios of great apes -- no grandmothering -- with modern-day human hunter-gatherer populations.

They found that over a million years, the ratio of available males to females ready to conceive doubled when 'grandmothering' was present, averaging about 111 males for every female.

Co-author Dr Peter Kim, at the University of Sydney, says the increase in human life span meant while older women became infertile after menopause, older men still remained able to reproduce.

Over time this created an imbalance in the sex ratio of fertile adults increasing competition among men for the still-fertile females, says Kim.

Increased competition led to reduced success in finding a mate, making it more important to find a mate and guard them.

The study answers the mystery, he says, of "why humans are such highly monogamous species while other species are not".

"Grandmothering first drove the change in human life histories and that caused there to be a shift in fertile male to fertile female ratios and that caused the next shift," Kim says.

"Suddenly males optimal behaviour is to link with one female and to stay and protect them and have all their kids with that female."